Direct conversion receivers function by mixing a desired radio frequency (RF) signal or intermediate (IF) signal down to baseband, or some very low frequency offset from dc. For direct conversion, the mixer's local oscillator (LO) frequency is approximately equal to the desired RF input frequency. Thus, the magnitude of the baseband dc signal is proportional to that portion of the RF signal that is exactly equal to the LO frequency. Any variations in RF power due to environmental (fading, multi-path) or circuit functionality will affect the dc voltage level at baseband.
The baseband signal of direct conversion receivers may include a parallel I and Q channel configuration, where Q is 90 degrees out of phase with I. Direct conversion receivers generate a residual dc offset error in the baseband I/Q paths due to LO self-mixing and interstage mismatch. The dc offset error manifests itself as an unmodulated, co-channel interferer exactly at 0 Hz on channel, the magnitude of which is directly proportional to the magnitude of the I/Q baseband path dc offset error. For modulation techniques where discrete time-sampled information is embedded in either phase, amplitude or both portions of the carrier frequency (digital modulation strategies), the bit error rate (BER) at sensitivity, minimum BER at strong signal conditions, and signal quality estimates (SQE) can be degraded by excessive dc offset errors. For modulation techniques where the continuous time-varying analog signal is embedded into either the phase, frequency or amplitude portion of the carrier frequency (analog modulation strategies), sensitivity degradation, squelch falsing, audio artifacts that degrade the demodulated audio quality, and even generation of audio tones in the absence of a desired signal (self-quieter) can be attributed to the presence of the co-channel interferer generated by dc offset errors.
Previous strategies to eliminate the undesired dc component utilize various high pass filter (HPF) topologies in either analog and/or digital subsections of the receiver. These strategies are implemented in algorithms in a digital signal processor (DSP) or in analog hardware and are known in the art. While these strategies successfully eliminate dc offset errors, the HPF response induces a spectral “notch” that distorts the received spectrum by eliminating the desired modulated information that also happens to fall within the HPF response. In extremely severe instances (wide HPF corner), the notch effect can degrade sensitivity, and audio quality and prevent carrier detect (sync) when receiving analog or digital modulated signals. For digital protocols, reducing the notch bandwidth to extremely narrow corners relative to the baud rate can mitigate these degradations; however, this can also force the HPF response time to be extremely slow relative to slot lengths and may cause missed slots (sync acquire failed) in dynamic RF environments (fading, multi-path, strong signal IM). For constant envelope, non-slotted modulation strategies (FM, SSB-AM) even narrow notch settings will induce undesired distortion effects that are not resolved by today's HPF implementations.
Accordingly, there is a need for a dc compensation strategy that eliminates the dc error for both digital and constant envelop modulation protocols, while being sufficiently flexible to provide both guaranteed slot acquisition and distortion free recovered audio.